DNA sequencing methods have remained largely unchanged in the last 20 years [Sterky and Lundeberg, ‘Sequence analysis of genes and genomes’, J. Biotechnology (2000) 76, 1-31]. The Sanger method is a well-known method of DNA sequencing, and comprises DNA synthesis, with termination of DNA replication at points of di-deoxynucleotide insertion. The DNA synthesis is followed by electrophoresis of the synthesised DNA to separate DNA molecules according to their mass to charge ratios, thereby allowing determination of the DNA sequence. A disadvantage of the Sanger method is that electrophoresis is complex, costly and hazardous. It is an object of the present invention to provide a sensing apparatus and method whereby Sanger-type sequence determination employing di-deoxynucleotide triphosphates can be carried out without need for separation of extended oligonucleotide strands. However, as indicated above, the invention can be applied more broadly to monitoring of any chemical event which will give rise to a fluctuation in ionic charge, e.g. proton release.